1. Field of the Invention
The present invention relates to an image recording apparatus such as a copier or a laser beam printer, and more particularly to such image recording apparatus capable of controlling the image recording conditions.
2. Description of the Prior Art
Image formation in an electrophotographic image recording apparatus such as a copier or a laser beam printer is generally achieved by uniform electrostatic charging of a photosensitive member with corona discharge, imagewise exposure of said photosensitive member to form so-called latent image which is composed of a charge pattern corresponding to an original image pattern, and rendering said latent image visible by depositing developer which is generally called toner.
Such latent image can be formed in various manners, for example by uniform charging of the photosensitive member followed by an exposure to a light image, or by uniform charging of the photosensitive member, then an exposure to a light image simultaneously with an AC charging or a DC charging of a polarity opposite to that of the first charging, and a flush exposure to light over the entire surface.
Also the above-mentioned image development into the visible state may be effected after so-called latent image transfer step, in which the latent image formed on the photosensitive member is transferred onto another latent image bearing member.
In any of the processes mentioned above, the latent image has to be maintained at an appropriate potential in order to match the image development.
Unless the latent image is maintained within a determined range for the image development of a determined condition, there may result an unstable image density and a background staining or fogging.
The maintenance of a constant condition, i.e. a constant potential in the latent image can be hindered for example by (1) the conditions of corona discharge varying according to temperature and humidity, (2) dependence of characteristics of the photosensitive drum on temperature and humidity, and (3) fluctuation among different photosensitive drums.
In order to avoid the influence of the above-mentioned factors, there is already proposed an electrophotographic recording apparatus in which the potential of a dark or light area on a photosensitive member is detected by a potential sensor and the high voltage for generating corona discharge is so controlled as to bring the detected potential toward a desired potential, and such apparatus is already found effective to a certain extent.
In the conventional potential controlling method, however, the image quality may not be maintained at the initial level during a continuous recording operation even though the initial image quality may be satisfactory, since the potential drifts away from the initial value.
For example, as a general tendency, the potential of a photosensitive member may become gradually higher or lower after repeated corona discharges even if the conditions of corona charging are maintained same, and such tendency is particularly marked when the image recording is conducted after the apparatus is put out of operation for a prolonged period.
Such tendency, if caused by the photosensitive member itself, for example by the charging hysteresis thereof, may be resolved by suitable selection of materials and manufacturing process of the photosensitive member, but the developmental work directed to such purpose will not only be time-consuming work but also be expensive, thus inevitably leading to the high cost of the photosensitive member.
Also a continuously stable image quality cannot be expected merely from the potential control before the start of recording operation, in case the temperature and humidify affect also to the potential.
The drawbacks of the conventional methods are well illustrated in FIGS. 1A and 1B, showing the change of dark potential in time, in case the latent image is to be formed in an exposed area.
FIG. 1A shows a case in which the dark potential gradually increases in the course of a continuous recording operation.
In such case the dark potential eventually exceeds an upper limit potential V.sub.ul in the course of a continuous recording operation, even if the initial dark potential V.sub.di is adjusted to an optimum value.
Also FIG. 1B shows a case in which the potential becomes gradually lower and eventually becomes lower than a lower limit potential V.sub.ll.
Said upper and lower limit potentials V.sub.ul, V.sub.ll are determined by the matching of the developing device and the image potential. As an example, in case the light potential is a smaller negative potential than the dark potential and the developer is so-called two-component developer composed of carrier particles such as iron powder and of toner particules made of carbon and resinous material, V.sub.ul corresponds to a limit potential not causing carrier deposition, and V.sub.ll corresponds to a limit potential not causing toner deposition in the back-group area.
Consequently a satisfactory image quality can only be obtained, in a continuous recording operation, up to a time t.sub.1 in case of FIG. 1A or up to a time t.sub.2 in case of FIG. 1B.
In order to avoid such drawback the potential control is indispensable during the course of a continuous recording operation.
In case of intermittent image forming operations, the potential control may be effected for each frame of image to continuously maintain an appropriate potential, but such method inevitably reduces the through-put of the apparatus.
It is becoming more and more necessary to obtain a stable image quality without affecting the through-put of the recording apparatus, since a higher process speed is required for recent electrophotographic copiers and computer output terminals.
The high voltage for generating corona discharge may be regulated according to the tendency of potential drift of the photosensitive member if such tendency is constantly predictable, but in practice such tendency varies depending on the manufacturing lot of the photosensitive members and on the ambient conditions.